Actively transmitting arrayed sensor systems, such as linear or phased array radar system, include multiple transmitting and receiving antenna elements that are arranged in space in the antenna array. For example, a phased-array radar antenna consists of a number of transmitting and receiving antenna elements arranged in space and each antenna element transmits radar signals and receives reflected signals (signal reflections) from targets as well as other items present in the environment. When a target is detected by the radar system, its position is estimated based on the received reflected signals at one or more of the antenna elements. In some radar systems, antenna elements are grouped to formed so called sub-arrays, and the received reflected signals of the group of elements (sub-array) are combined in specific ways to form sub-array signals which are used for detection and estimation of a target. Radar target positions are usually identified by the range, azimuth angle, elevation angle, and Doppler frequency shift space. However, the range resolution, angular resolution, and Doppler resolution are limited by the radar system's signal bandwidth, radar aperture, and signal pulse repetitions. When multiple targets are present in a single radar range, angular, and Doppler resolution cell, the individual targets cannot be distinguished without further processing.
In some environments, signals are transmitted on the same frequency as the transmission frequency of the radar causing interference with the radar reception of reflected signals. These signals may have known waveforms or unknown waveforms, and the transmission of these signals may be inadvertently interfering with the signal reception of the radar or may be deliberately transmitted to interfere with reception of the reflected signals (i.e., jamming the radar receivers) to prevent the radar from detecting and estimating the position of targets.
To overcome interfering signals transmitted by other signal sources and/or jamming signals from radar jammers, some radars will manipulate the array radiation pattern to suppress the interfering and/or jamming signals based on the detected locations of the jammers by its Electronic Support (ES) systems. Other radars use adaptive beam forming techniques that adjust complex weights being applied to element or sub-array signals to suppress the interfering and/or jamming signals without explicitly locating the jammers while extracting the desired target signal. In some environments, the interfering signals and/or jamming signals are also reflected by objects in the environment, resulting in multipath signals arriving at the receivers of the radar. These replicas of the transmitted signals are “coherent,” which means they are delayed and complexly attenuated versions of the original transmitted signals, which causes the signal source location estimation process to be even more difficult and require more processing resources.
Since adaptive beam forming signal processing for a sensor system typically involves the use of the covariance of the element signals or sub-array signals, the typical adaptive beam forming signal processing cannot resolve for a location of a signal of interest because coherent interference received with the signal of interest cannot be separated in the Eigen space of the signal covariance matrix. This causes the adaptive beam forming signal processing to be unable to resolve the DoA of the signal of interest from the associated coherent interference including multipath signals arriving at the signal receiving elements of the sensor, and results in the sensor system being unable to estimate the location of the signal of interest.
What is needed is a sensor system that can determine a location of a target, even resolve locations of multiple targets within a defined area, such as a single range-angular-Doppler detection cell for a radar system, or an area bounded in elevation and azimuth for passive signal receiving systems, in environments including a high level of signal interference including multipath signals, and may include jamming signals and associated multipath signals or repeated signals.